1. FIELD OF THE INVENTION
The subject invention relates to electronic displays and to control circuitry for controlling the light output of these displays.
2. DESCRIPTION OF PRIOR ART
In the more common type of brightness control circuit for electronic displays, the current supplied to the display elements is controlled as a function of the amount of illumination desired from the display. This is normally done by adjusting a resistance through which the energizing current flows, or by adjusting the supply voltage as applied through an emitter follower circuit. In addition to being inefficient and wasteful of energy, these forms of control have a limited range over which the illumination can be uniformly controlled and tends to turn off completely at low brightness levels. As a related matter, the control circuit may be subject to temperature instabilities and excessive variations in component tolerances, giving rise to a nonuniform illumination from the display elements.
In addition to the foregoing, the public is aware of U.S. Pat. No. 4,090,189. This patent discloses a brightness control circuit for controlling the current flow from a source of energizing potential to an L.E.D. electronic display, the output of said potential source being coupled through a transistor switching means for supplying pulses of approximately constant peak current to the display elements. The transistor switching means is controlled so as to provide a periodic on/off operation having a duty cycle that is varied to control the brightness of the display. The operation of the transistor switching means is controlled as a function of a drive signal of approximately constant peak voltage derived from a capacitive charge-discharge circuit. This circuit includes a capacitor that is charged through a serially connected charge circuit means which includes a brightness control resistor whose resistance is adjusted for a selected condition of brightness to determine the initial rate of charge of said capacitor voltage, the capacitor being periodically and briefly discharged through a discharge transistor. During the charge time the capacitor voltage is made to exceed a given threshold voltage Vth, and during discharge the capacitor voltage is reduced toward a reference level that is below Vth. A threshold voltage sensing transistor having its input coupled to the capacitor through a resistor voltage divider circuit and its output coupled to the input of the transistor switching means responds to the voltage across the capacitor and derives at its output a drive signal having a duty cycle that is dependent upon the relative time said capacitor voltage is above and below Vth. Thus, the threshold voltage sensing transistor provides the transistor switching means with a precise on/off operation.